1. Field of the Invention
The present invention pertains to a catalytic material having improved conversion performance and, in particular, to catalytic materials comprising a support coating comprising a high porosity support material.
2. Related Art
It is well-known in the art to utilize oxidation catalysts, including those commonly referred to as three-way conversion catalysts ("TWC catalysts"), to treat the exhaust gases of internal combustion engines. Oxidation catalysts promote the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") in engine exhaust to H.sub.2 O and CO.sub.2. TWC catalysts promote such oxidation reactions and also promote the substantially simultaneous reduction of nitrogen oxides ("NO.sub.x ") in the exhaust to N.sub.2. It is well-known that successful functioning of the TWC catalyst to promote oxidation of HC and CO and to substantially simultaneously reduce NO.sub.x requires that the engine be operated at or close to stoichiometric air/fuel conditions.
It is also well-known in the art to provide such catalysts in the form of a catalytic material comprising a refractory inorganic oxide support material, e.g., activated alumina, on which is dispersed a catalytic metal component such as a platinum group metal component. The catalytic material is normally provided as a thin coating or "washcoat" adhered to a refractory carrier substrate. The latter often takes the form of a body made from a suitable material such as cordierite, mullite or the like, which is formed to have a plurality of parallel, fine gas flow passages extending therethrough. Typically, there may be from about 100 to 600 or more such gas flow passages per square inch of end face area of the substrate.
U.S. Pat. No. 4,757,045 to Turner et al, dated Jul. 12, 1988, discloses a catalytic material comprising a platinum group metal component dispersed on a support coating. The support coating comprises two portions of refractory metal oxide materials. The first material has a surface area greater than about 25 m.sup.2 /g, an accessible pore volume of greater than about 0.03 cubic centimeters per gram (cc/g), and a pore size range such that at least 95 percent of its pore volume is from pores having diameters of less than 2000 .ANG.. The second material comprises about 1-20 percent of the support coating, has a surface area of less than 25 m.sup.2 /g, an accessible pore volume of greater than about 0.03 cubic centimeters per gram (cc/g), preferably, 0.1 to 0.3 cc/g, and a pore size range such that at least 35 percent of its pore volume is from pores having a diameter of 2000 .ANG. or greater when measured at a particle size of at least 44 micrometers (microns) diameter. The second material is obtained by the comminution of previously manufactured ceramic-based catalyst members, e.g., ground ceramic honeycomb carriers coated with catalytic material (referred to herein as "scrap catalyst material"), while the first material can be a conventional, stabilized metal oxide powder such as stabilized alumina. While Turner et al characterize the second metal oxide as having a greater pore volume than conventional alumina (see column 4, lines 21-27), the second metal oxide is a low porosity material and the first metal oxide is a medium porosity material relative to the present invention as described below.
In Heterogeneous Catalysts in Practice, by Charles Satterfield, published by McGraw-Hill Book Co., it is shown that the pore (Knudsen) diffusion coefficient is proportional to the pore radius of porous solids (page 377) while the diffusion flux (mol/(cm.sup.2 of pore area.multidot.S)) of hydrogen gas and nitrogen gas increases by about two orders of magnitude as pore radius increases from about 1 nanometer (nm) (10 .ANG.ngstrom units) to about -100 nn (1000 .ANG.). This indicated the benefits of appropriate pore size distribution inside porous solid for heterogeneous catalysis. Therefore, other gases such as NO.sub.x, CO, and some hydrocarbon molecules are expected to behave similarly although the "mass flux and pore-radius relationship" curves may shift up or down as molecular weight and structure vary.